Peeling away Mercedes' slimline sidepods

An updated Mercedes W13 with radically narrow sidepod inlets broke cover in Bahrain testing. It created a considerable stir.

There is something very ‘stealth fighter’ in the way the Merc’s bodywork deflects into different planes in this region, with odd unresolved-looking angles. But in this case it isn’t evading radar; rather it’s cheating the air, tricking it into providing downforce and cooling but at less expense in drag. Furthermore, that tall, narrow inlet has allowed the regulation upper impact structure to be exposed and sheathed in a cover which directs airflow from above down to the outer floor area.

Traditionally, this part of the leading edge of the sidepod will feature a big undercut in its lower regions with which to create a low pressure to hurry the air around that obstructive corner. That’s how it was on the original Merc W13 which ran in the Barcelona shakedown a week earlier. The new layout pretty much inverts that tradition, a narrow top and wider bottom visually recalling some of the ‘triangular’ monocoque shapes of the mid-1970s such as the Brabham BT42/44 and BRM P201. But in those cases the triangular section ran the length of the tub and was about minimising the centre of gravity height, when full, of the fuel tanks which were housed in there. This time it’s nothing to do with centre of gravity, but of finding a new optimised trade-off between the demands of downforce, drag and cooling under the new venturi floor regulations introduced this year.

The wide bottom of that section of bodywork around the radiator inlet also allows maximum aerodynamic use to be made of the exposed upper side impact structure – through a dimensional ‘shadow’ requirement in the regulations. It allows that contoured impact structure to extend further outwards. If the bottom bodywork was narrower there, that side impact ‘wing’ would be obliged to be similarly narrower.

Getting the best combination of airflow through the tunnels and around the sidepods is the key to maximising downforce. There’s only so much airflow mass to work with and how much goes around and how much goes through is crucial because the two flows then meet up again at the rear of the car. The two flows can be thought of as one which is initially split then later re-connected. Getting the best overall performance from that combined flow is what the aerodynamicists are striving to do.

The airflow going around the sidepods exits over the top of the venturi tunnel diffuser and beam wing and in that way draws on the underbody flow, effectively scavenging it out of there faster. On its way around the sidepods that airflow is also enhancing the underbody’s performance by sealing the floor with vortices. The pressure they create around them keeps the airflow through the tunnel from leaking from the sides and stops the floor’s low pressure drawing in turbulent air from outside. This all helps to maximise the underfloor air speed – and the greater that speed, the greater the downforce.